DESCRIPTION (Principal Investigator's Abstract): The long-goal of this project is to understand the mechanisms that may link physical activity and growth in children. Growth hormone (GH) is released in response to acute exercise and is important for substrate regulation, but there is increasing evidence that GH stimulation during physical activity may modulate the growth process as well. Episodes of vigorous physical activity occur naturally in healthy growing children, but whether or not these spontaneous episodes act to influence growth remains unknown. The following hypotheses will be tested: 1. GH in children is released in response to exercise only when a definable threshold of exercise intensity has been exceeded. 2. Children compared to adults will demonstrate a greater GH response for an exercise stimulus of the same relative intensity. 3. In adults and children, exercise will stimulate increases in serum IGF-1 or binding protein. This may result from generation of IGF-1 induced by GH, or, alternatively, by a GH-independent effect of exercise. 4. Patterns of physical activity in children are characterized by short bursts of high-intensity activity. Significant numbers of these episodes represent a sufficient metabolic stimulus to elicit a GH response. A team of pediatricians, physiologists and anthropologists has been assembled to integrate studies of GH and exercise under laboratory conditions with studies of patterns of activity and energy expenditure that occur in the field, in the real lives of children. These hypotheses will be tested using breath-by-breath measurements of gas exchange during exercise; measurements of hormonal and metabolic response to exercise (e.g., GH, IGF-1, IGF-1 binding protein, catacholamines); stable isotope techniques for measuring energy expenditure under field condition (doubly-labelled water and 13C labelled bicarbonate); and direct observational measurement of patterns of activity. The results of these efforts will be used to establish guidelines for optimizing the role of physical activity in the lives of children with chronic diseases.